Sensors 2026 , 26 , 2049
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Office (printing and writing) paper must also comply with certain specifications. Although they are not as strict as those for banknotes and security is not an issue, the final product must fulfil the functionality requirements. Office paper whiteness is an important parameter; thickness and grammage (paper weight per square meter, gsm) are also considered. Humidity must be controlled, and fillers and sizing agents must have been added and controlled during production so that resistance to wetting is ensured and ink running (bleeding, feathering) is prevented. Office paper has several utilizations, both at home and in the office; it is more commonly used for graphic printing. Therefore, the ink and printing methodology may vary, and handwriting should also be taken into account. Although a high degree of consistency in quality is required, variability may arise from wood fibers (as different plant species may be used in their production), from additives used in the paper-making process, and from other factors. Among the trees used for office paper, Eucalyptus globulus , birch, aspen, oak, and acacia are mostly used [1]. They are all sources of hardwood fibers (with fiber lengths ranging from about 0.5 to 1.5 mm), which are shorter and thinner than softwood pulp fibers, and provide a smooth printing surface with high opacity [2]. Generally, hardwood fibers contain more cellulose and hemicellulose and less lignin than softwood, while softwood has a higher proportion of extractives, i.e., resin. Eucalyptus is the most common wood used for pulp production in Portugal. Acacia is a strong competitor in Asia due to propitious ecological conditions. Compared with Eucalyptus globulus , pulp yield from acacia can be higher and residual lignin content lower [3]. In addition, light scattering, which is important for writing and printing papers, was shown to be higher with acacia wood thanwith E. globulus , although eucalyptus fibers, at a given density, did show superior strength [3]. In Finland, Metsä produces birch-bleached hardwood kraft pulp (BHKP) [4]. It is also common to use blends of hardwood and softwood pulps to meet the desired paper properties. Paper traceability is important in forensics, audits and certifications. Publishers need to ensure uniformity in re-editions and printing quality. Country of origin determines customs duties, and environmentally concerned citizens and ONGs need to prove that the wood does not come from protected forests and they are interested in assuring the inclusion of a minimum percentage of recycled paper. Paper is ordinarily evaluated by its physical properties: density, air permeability, smoothness, stretch, stiffness, tensile index, opacity, and others [3]. Although office paper seems odourless, it can release volatile compounds originating from the wood itself and from the pulping, bleaching, and paper-making processes, including additives and fillers. Gas chromatography (GC) coupled with flame ionization detection (FID), electron capture detection (ECD), and mass spectrometry (MS) has been used to detect and identify volatile organic compounds (VOCs). These methods require expensive equipment, specialized operators, and a large database to compare the compounds. These techniques have al- ready been used to study VOCs emitted from historical papers and old documents using SPME [5–7] or SKC passive (diffusive) samplers or “badges” [8]. However, as far as we know, there has been no research on new paper sheets that emit no perceptible VOCs and are not considered a cause for concern. An electronic nose (e-nose) based on an array of piezoelectric quartz crystals was already used to analyze historical papers [9]. The method used was not only non-destructive but also inexpensive. A tailored electronic nose could help to identify distinctive VOC profiles from different papers by selecting an appropriate sensitive membrane for each sensor in the array. According to Gardner and Bartlett [10], an electronic nose “is an instrument, which comprises an array of electronic chemical sensors with partial specificity and an appropriate
https://doi.org/10.3390/s26072049
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